In prior-art systems, the steel frame requires reinforcement, i.e. additional thickness, in order for the screw fastening to be sufficiently sturdy. In these systems it is necessary to use a plurality of countersunk screws, the opening and fastening of which in connection with servicing is cumbersome and very time-consuming. When the blade is of the same material in the area of the reinforcement, it reacts immediately to heat, which in turn causes distortions in the actual blade. The prior-art systems in which there are reinforcements are also very expensive.